1) Field of the Invention
The invention relates to composite materials based on titanium diboride and processes for their preparation.
2) Background Art
Materials based on titanium diboride have an extremely high melting point, good chemical stability, great hardness at low and high temperatures, good creep resistance and oxidation resistance, very good electrical conductivity and a low density. Owing to these properties, they are suitable for a large number of industrial uses, for example as cutting tools, as electrode material but also as wearing components or as structural parts in machines subjected to high thermal stress.
The preparation of such composite materials presents problems, since TiB.sub.2 can be compacted only to an insufficient extent and only at extremely high temperatures of more than 2100.degree. C. by sintering under atmospheric pressure. It was therefore attempted to decrease the sintering temperature and at the same time to increase the compactability by adding sintering assistants.
U.S. Pat. No. 4,379,852 by Watanabe et al. describes combinations based on diborides, in particular of titanium and of zirconium, of binders based on boride, such as, in particular, Co, Ni or Fe boride, and their preparation. The substances described in the patent are very hard and solid but particularly brittle.
U.S. Pat. No. 4,259,119 by Watanabe et al. describes composite materials consisting of 99.99 to 70% by weight of a combination of two diborides selected from the group comprising the diborides of Ti, Ta, Cr, Mn, Mo, Y, V, Hf, Nb, Al and Zr and 0.01 to 30% by weight of a binder based on boride, which is selected from one or more of the metal borides of Ni, Fe or Co. In the Examples, only NiB, Ni.sub.4 B.sub.3, FeB and CoB are used as boride-based binders.
In both of the above-mentioned patents, Fe.sub.2 B is also mentioned as one of the possible binders for the preparation of the particular composite materials. However, none of the examples describes a material which contains Fe.sub.2 B as a sintering assistant.
R. Telle and G. Petzow, Mat. Sci. Eng. A105/106 (1988), pages 97-104, disclose that composite materials which, in addition to titanium diboride, also contain metal-rich borides, such as Fe.sub.2 B, Ni.sub.3 B or Co.sub.3 B, have low hardness and toughness. For this reason, and also on the basis of the prior art stated below, it has always been attempted in the past to avoid the formation of Fe.sub.2 B in the particular composite materials. Thus, for example, R. Kieffer and F. Benesovsky, Hartmetalle S476, Springer Verlag 1965, class such materials as brittle and not very promising for use as cutting and wearing materials. Furthermore, in the publication by I. Smid and E. Kny "Evaluation of Binder Phases for Hard Metal Systems based on TiB.sub.2 ", Int. J. Refractory and Hard Materials 8, 1988, 135-138, a weakening of the binder phase and hence of the material is attributed to the occurrence of Fe.sub.2 B (page 137, 1st paragraph, lines 9-11). In the patent application EP-A-0 433 856, which describes hard metal mixed materials based on borides, nitrides and iron binder metals, it is stated repeatedly that the formation of Fe.sub.2 B must be avoided (cf. EP-A-0 433 856, page 2, lines 14-19 and lines 41-45; page 3, lines 23-32). In spite of the mention in a list in the above-mentioned U.S. Patents, a person skilled in the art will therefore tend to avoid experiments with Fe.sub.2 B as a sintering assistant for materials based on titanium diboride, in view of the further stated publications.
It is therefore an object of the present invention to provide highly dense, very hard, tough and solid composite materials which are based on titanium diboride and can be sintered even at relatively low temperatures under atmospheric pressure.